For numerous orthodontic procedures casts are made of the teeth from both the upper and lower jaws. These dental casts are then mounted to a mounting plate which is attached to a dental articulator to reproduce the location and movement of the upper teeth and maxilla, relative to the lower jaw, and mandibular teeth. Many of these prior art articulators are used in making artificial dentures and inlays or crowns and are known as gnathological articulators. Other articulators are used in oral surgery during which, typically, sections of bone or other tissue are removed to alter or shift the relative relation between the upper and lower jaws.
In many of the prior art articulators the lower jaw remains fixed and cannot be moved relative to the hinge axis of the articulator; see U.S. Pat. Nos. 3,159,915; 3,590,487; 4,058,895 and 4,185,387. This is a disadvantage since the true operable relationship between the jaws cannot be accurately modeled for surgical techniques in which the lower jaw is altered in size and shape.
Other prior art articulators attempt to simulate the rather complicated movements of a human mandibular joint. This type of gnathological articulator allows the user to view the interplay between artificial teeth so that appropriate adjustments can be made. See, for example, U.S. Pat. No. 4,047,302. Other related devices are directed to apparatus for the measurement and recording of jaw movement, see U.S. Pat. No. 3,815,242, and are used in fabricating dentures, bridges and the like.
Some of the prior art articulators have recognized the importance of being able to independently move both the upper and lower dental casts. However, they typically do not provide all six degrees of freedom, three translational and three rotational, for both dental casts. If all six degrees of freedom are provided, then the three rotational degrees of freedeom are generally provided via a universal or ball joint; the individual rotational movements about three orthogonal rotational axes are therefore not directly readable from the apparatus. See U.S. Pat. Nos. 4,083,114; 4,128,942. Another patent which may be of interest is U.S. Pat. No. 2,959,857.
An exemplary surgical procedure using a prior art surgical dental articulator is a mandibular advancement. Before the surgery, a surgical splint, which is a U-shaped piece of plastic material, must be constructed for placement between the patient's upper and lower dental arches after the surgery. This splint is used to properly position the lower jaw to the upper jaw which are wired together via the teeth while the bone tissue heals.
Preparatory to making the splint, the patient's dental casts are mounted to an articulator, such as the articulator made by SAM of Munchen, West Germany. For lower jaw surgery, the articulator is held in a protrusive or retrusive position to construct the splint.
For upper jaw surgery, the base of the plaster cast is made quite thick so that a pattern of grid lines can be etched on the base of the cast to serve as a reference. The amount of bone to be removed from the patient is determined from head X-rays using standard techniques. For example, assume that 7 mm of bone is to be removed from the posterior or rear portion of the upper jaw and 4 mm from the anterior or front portion of the upper jaw. Using the etched grid lines as a guide, a wedge of material 7 mm at the rear of the cast and 4 mm thick at the front of the cast is removed from the base of the upper plaster dental cast. This is typically accomplished using a hand saw or a circular cutter. The separated portions of the dental cast are then joined back together using a thin mix of mounting plaster and the dental casting is remounted on the articulator.
The pliant U-shaped splint material is placed between the teeth of the dental casts and impressions of the occlusal surfaces of the teeth are formed in the splint material. The splint material can be an acrylic in which a monomer and a polymer are mixed to produce a plastic, dough-like material which is formed into the U-shape. The splint material is hardened by placing it, while still between the teeth of the dental casts on the articulator, in a proper environment for the particular material used. One type is cured or hardened by placing in a warm, pressurized environment.
After the removal of the wedge of material, the teeth on the articulator should be in the same relative position as the teeth of the patient after the removal of the bone. If this is so, the indentations in the surgical splint created by the teeth of the plaster dental casts will be identically positioned as the patient's teeth will be after surgery. This is important because patient's jaws are wired shut for several weeks or months while healing.
Two basic shortcomings exist with the above-described procedure. First, if one uses an articulator in which the lower dental cast is fixed, the upper dental cast must be moved even though the lower jaw is the one on which the surgery is performed. Therefore the positions of the dental casts relative to the hinge axis will not be correct. Second, the procedure for scoring grid lines on the mounting plaster on the base of the dental cast, plotting the tissue to be removed, removing sections from the base of the dental cast, and then bonding the severed sections of the dental cast back together is time consuming and cumbersome.
Therefore, what has been missing in the prior art is a surgical articulator which can be used by the orthodontist or oral surgeon to independently move either or both of the dental casts a measured amount in any individual translational or rotational direction in each plane of space.